1. Field of the Invention
The present invention relates to an optical fiber suitable for a wavelength divided multiplex transmission, an optical fiber module using the particular optical fiber, and an optical amplifier.
2. Description of the Related Art
Wavelength division multiplexing transmission system has been studied in 1.55 μm wavelength band, which is a gain band of an erbium-added optical fiber, and the non-zero dispersion shifted optical fiber (NZ-SF) having about 4 to 8 ps/nm/km of dispersion in the 1.55 μm waveband is commercialized for an optical transmission line. Incidentally, the 1.55 μm wavelength band is generally called C-band and approximately ranges between 1530 nm and 1570 nm.
In order to realize a high quality wavelength division multiplexing transmission, it is important to suppress the self phase modulation (SPM) and the cross phase modulation (XPM), and necessary to expand the effective area of the optical fiber for the suppression. Therefore, the NZ-DSF having large effective area has been studied, for example, OFC'96 WK15 and OFC'97 TuN2.
However, larger effective cross sectional area of the NZ-DSF may result in increasing the bend loss and the dispersion slope.
Under the circumstances, extensive research has been conducted in an attempt to overcome the problem noted above by optimizing the refractive index profile and manufacturing conditions for a cable. However, it is still a difficult problem to reduce the dispersion slope. Particularly, in the high speed transmission not lower than 40 Gb/s, recently vigorously studied, the increase in the dispersion slope has become a serious problem to be solved.
Also, the wavelength division multiplexing transmission region is studied to be expanded into L-band (the wavelength range of between 1570 nm and 1610 nm).
Incidentally, the NZ-DSF has a dispersion of approximately 4 to 8 ps/nm/km at the wavelength of 1.55 μm, and a positive dispersion slope. When the absolute value of the dispersion is excessively small, the transmission characteristics caused by the four wave mixing (FWM) deteriorates. In order to suppress such deterioration, the dispersion of the NZ-DSF in the 1.55 μm wavelength band is not set at a very small value.
Under the circumstances, it is proposed to compensate the dispersion and the dispersion slope of the NZ-DSF in the 1.55 μm band by using a dispersion slope compensating fiber (DSCF). Various proposals have been made since the particular technology was reported in ECOC '96TuP. 1.
However, the above DSCF was proposed to compensate the dispersion slope of the NZ-DSFs having the dispersion characteristics as shown “b” and “c” in FIG. 1.
However, the compensation of the dispersion slope has not been proposed in the past for NZ-DSF having a large dispersion slope as denoted by “a” shown in FIG. 1. Particularly, the study was scarcely made on the optimization of the compensation of the dispersion slope in the L-band of the NZ-DSF having a large absolute value of the dispersion slope.
Incidentally, the straight lines “a”, “b”, “c” shown in FIG. 1 denote the dispersion characteristics, where the dispersion at the wavelength of 1550 nm is set at 5 ps/nm/km. The NZ-DSF denoted by “a” has a dispersion of about 10 ps/nm/km in the L-band. It was difficult to compensate for the dispersion by the DSCF proposed above.
It should also be noted that the DCF has a high non-linearity and is expected for a Raman amplifying medium. However, sufficient study has not yet been made concerning the optimization thereof.